526 ADAM SEDGWICK. 



{b) If so, how did this ancestral form itself arise? 



If the answer to the latter question falls within the province 

 of the ^astrgea theory, Balfour did not accept that theory and 

 MetschDikoff is wrong in saying that he was strongly inclined 

 towards it. If, on the other hand, the gastraea theory simply 

 generalises from a large number of anatomical and embryo- 

 logical facts as to the past existence of an animal with a parti- 

 cular structure, and leaves the question of origin out of consi- 

 deration, then Balfour undoubtedly did accept the gastrsea 

 theory, but did not thereby, as Metschnikoff seems to think 

 must necessarily have been the case, reject the Parenchymella. 

 On the other hand, Balfour expressed himself distinctly in 

 favour of the latter, though he did not call it by that title, 

 for does not he say, and does not Metschnikoff quote him as 

 saying (No. 23, p. 156), that he thought it probable that the 

 ancestors of Coelenterates possessed a solid endoderm of 

 amoeboid cells ? 



This is not the only point in which Metschnikoff has mis- 

 understood Balfour's views. On p. 141 of No. 23, he ascribes 

 to him the view that an amphiblastula form would represent 

 more nearly than any other the transition between the Pro- 

 tozoa and Metazoa. Balfour maintained no such position, as 

 has been already pointed out by the translator of Metschnikoff's 

 paper on the " Intracellular Digestion of Invertebrates," 

 ' Quarterly Journal of Microscopical Science,^ vol. xxiv, p. 107, 

 note. 



But to resume : Metschnikoff supposes that the parenchy- 

 matous ancestor was preceded by a hollow spherical form, the 

 blastula, the cells of which were all alike ; and that the blastula 

 became a parenchymatous gastrula by the migration inwards 

 of cells from its external wall. 



I do not understand on what grounds Metschnikoff is so 

 strongly disposed towards the view that the hollow blastula 

 represents a primitive form. 



There are many cases, even amongst the Coelenterates, in 

 which a hollow blastula is not formed and segmentation gives 

 rise to a solid planula, and in the rest of the animal kingdom. 



